This pilot randomized study of approximately 49 patients (representing a total of 80 brain lesions) should be sufficient to explore the effect of margin expansion around the GTV for brain metastases. The patients enrolled in this study will undergo radiosurgery to a planning treatment volume generated by expanding the GTV of each lesion by either a 1mm or 3mm margin. Each lesion, not each patient, will be randomized to receive either the 1mm or 3mm margin with 40 lesions randomized to each arm (a patient with multiple lesions potentially could have a lesion randomized to the 1mm arm and a different lesion randomized to the 3mm arm). The primary outcome of 12-month local recurrence and the secondary outcome of the rate of radionecrosis at the SRS treatment site will be lesion-specific outcomes. All other secondary outcomes will be patient-specific outcomes.